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New Method for Counting, Analyzing Tiny Particles

Duke University engineers have developed a novel “nozzle” that permits a decades-old technology to detect tiny particles such as cells or viruses or to process even smaller samples without clogging.

In use in different forms since the 1950s, Coulter counters measure tiny particles suspended in a fluid as they pass through a sensing opening, or aperture, carrying an electrical current. The particles disrupt the current, yielding characteristic fingerprints that scientists can interpret.

These are standard machines in use at almost every hospital and most research laboratories in the country. To date, the size of particles that can be analyzed by the machine has been limited by the size of the aperture through which the fluid must pass.

The Duke engineers developed a totally new approach in which the solid and rigid aperture is replaced by a fluid “tunnel” that appears much like the lower end of a tornado’s funnel cloud. Using this “cone-jet nozzle,” researchers will not only be able to analyze much smaller particles, but sidestep the major problem with traditional Coulter counter apertures – they tend to clog up with impurities and must be replaced.

“This new approach can ‘swallow’ impurities much like a snake swallows its prey,” said Chuan-Hua Chen, assistant professor of mechanical engineering and materials science at Duke’s Pratt School of Engineering. “This would make the technique immune to clogging, which is a major concern, since the apertures used in Counter counters can be very expensive. Our aperture can be made much smaller than its conventional counterpart, because we do not have to worry about clogging.”

The results of Chen’s research, which is supported by the National Science Foundation, was published in the journal Physical Review X.

Currently, the smallest aperture available for a Coulter counter is about 20 microns, which means that the smallest particle that can be analyzed is about one micron, Chen said, adding that the cone-jet aperture developed by his group has a diameter of about 10 microns and is flexible enough to accommodate particles of a variety of sizes.

“Although the proof-of-principle has been demonstrated by the detection of micron-sized particles, we believe we can measure sub-micron particles because the cone-jet technique can produce an aperture diameter down to ten nanometers,” Chen said. This means nanometric particles can be analyzed using the new technique.

“The development of non-clogging Coulter counters has major practical implications, particularly given the potential to extend the analysis range down to even smaller, nanometric sizes,” Chen said. “At the same time, we can overcome the frustrating and costly clogging issues that plague Coulter counters.”

The clogging problem encountered by Coulter counters are usually caused by dust in the air, impurities in the water, or by the accumulation of biological matter in the samples being tested.

The Coulter counter, invented more than 50 years ago, provided the first high-throughput, standardized method to count and size cells and colloids. It has led to major breakthroughs in science, medicine and industry. In fact, the counter touches everyone’s daily life from having a blood test, to painting a home, from drinking beer to eating chocolate, swallowing a pill or applying cosmetics. It is critical to toners and ceramics as well as space exploration, where NASA uses it to test the purity of rocket fuel.